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Slide Rules

Circular Slide Rules

Slide rules that are round offer the length of a 10" rectangular slide rule in a pocket-sized (roughly 3.6") format, since the scales are on the circumference of circles. The scales are also continuous, so there is no need to make adjustments, such as folded and inverse scales, for results of calculations that go off the ends of the scales. Furthermore, these instruments are relatively easy to construct: the scales are printed on one or more disks, and the disks or a single disk and cursor are fastened together with a pin at the center. However, this simple construction is also not very durable, and so circular slide rules may get out of position and thus they lack accuracy, compared to linear slide rules with slides that move along carefully grooved channels.

This collection suggests the diverse appearances and functions of circular slide rules that were manufactured between the mid-19th and late 20th centuries. For example, before Mannheim-type linear slide rules became popular in the late 19th century, American inventors patented a variety of circular designs. Some circular slide rules were made to look like pocket watches, while others were intended to promote particular businesses—Whitehead & Hoag and Perrygraf were especially influential American manufacturers of promotional items. Inventors and makers such as Albert Sexton, Louis Ross, Claire Gilson, Norman Albree, and Ross Pickett wanted their circular slide rules to compete with linear instruments in the engineering and education markets. Other circular slide rules were designed specifically for surveying, such as stadia computers, or for navigation, such as Dalton instruments that may also be seen in the Smithsonian's exhibition, Time and Navigation. Even more specialized in purpose were slide rules for grading earthworks, determining the effects of nuclear bomb explosions, writing efficient computer programs, and betting on horse races.

Aeronautical engineer George Norman Albree (1888–1986) designed airplane components and slide rules. This is an example of his Duplex slide rule, which he intended to be used by grade school children. It is printed on both sides of a cardboard circle and consists of a logarithmic spiral scale for numbers from 10,000 to 32,100 on the front or "A" side and numbers from 31,170 to 100,000 on the back or "B" side. A rotating clear plastic indicator wraps around both sides.

The slide rule is a device to assist in multiplication, division and other mathematical operations. Invented in the 1600s, it became popular in American science and engineering in the 1890s. By the 1930s, slide rule use was taught in high schools.

From 1962 until 1972, Harvard University faculty cooperated with others in developing a humanistically oriented high school physics course that might attract more students to the subject. Staff developed not only textbooks, handbooks, transparencies and film loops but this extremely simple and inexpensive plastic slide rule.

The instrument has two circular logarithmic scales for multiplication and division (most elementary slide rules also had scales for taking squares and square roots). There also are linear scales of inches and centimeters.

A stylized bubble chamber image, the logo of Project Physics, appears over the rule. The slide rule was designed so that "Harvard Project Physics" showed just over the shirt pocket of a boy carrying it. This design may reflect the fact that there were no female undergraduates at Harvard College at the time. Not long after this slide rule was made, inexpensive pocket calculators displaced the slide rule.

This white plastic circular slide rule is on a rectangular base that has a 4-inch ruler on the left side and a 10-cm ruler on the right. A black plastic tape attached along the top edge reads: MATLACK. The top of the base is marked: CONCISE (/) SCIENCE TABLES (/) AND CIRCULAR SLIDE RULE. The slide rule has a D scale along the outer rim. A rotating disc fastened with a metal grommet has C, CI, L, A, S, T, and K scales. There is a clear plastic rotating indicator, which is labeled in red with the letters for the scales. The bottom of the base is marked: BY SAMA & ETANI.

The instrument fits in a rectangular black plastic case. There is no instruction manual, but the instrument likely was originally accompanied by a copy of: Sama & Etani, Reference Tables and Circular Slide Rule (Groton, Mass., 1969), http://sliderulemuseum.com/Manuals/M219_refC36_Concise_SamaEtani_ReferenceTables.pdf.

The donor, Glenn Matlack, purchased this slide rule in the fall of 1968 for his junior high school general science course at Tower Hill School in Wilmington, Del. Sama & Etani designed and distributed several slide rules made by Concise. For other slide rules by Concise and the company history, see 1985.0636.02, 1996.0141.01, and 2003.0012.01.